Engineering interfacial modification on nanocrystalline hematite photoanodes: A close look into the efficiency parameters

https://doi.org/10.1016/j.solmat.2019.110377Get rights and content

Highlights

  • Enhanced efficiency of mesoporous hematite photoanode designed via spin-coating deposition of polymeric precursor solution.

  • Sn-over hematite photoanode enhanced the charge separation however, with the cost of creating surface states.

  • Ni/Fe based materials suppress the surface states by passivation.

  • Interfacial engineering revealed the role of different modifiers on catalytic and separation efficiencies.

Abstract

This work describes an approach to direct evaluate the role of selected modifier added over hematite photoanode interfaces for light assisted water splitting. Hematite photoanodes with thickness varying from 25 up to 130 nm were designed via spin coating deposition of polymeric precursor that provides mesoporous planar morphology. The photoelectrochemical data analyzed in terms of global efficiency parameter (ηglobal) revealed a close dependence with hematite thickness for unmodified photoanode, which large number of interfaces associated with high charge recombination rate led to drop the electrode performance. To address this issue and manipulate the large number of interfaces due to the planar morphology, Sn4+ ions (1 and 3% molar ratio) were deposited over the hematite surface leading to enhance the electronic transport with a cost of creation of surface states at the solid-liquid interface. Then, a second modification process was done by photoelectrodeposition of Ni/Fe based material to suppress or minimizing the effect of surface state created by the Sn4+ ions addition. The results show that the interfacial traps were mitigated for the thinner hematite photoanodes, where the presence of Sn4+ and Ni/Fe significantly increases the ηglobal. The present find, highlights the importance of morphology design that allows a combination of modifier insertion to work in synergy for enhancing the overall photoanode performance.

Introduction

The increasing demand of energy of the modern society and the concern on limitation of the fossil fuels sources has encouraged the research on renewable energy sources. Among several alternatives, the hydrogen (H2) production has gathered great attention being considered one of the most promising clean and sustainable future energy sources [[1], [2], [3]].

One with the most elegant and clean solution for H2 production includes the process involving photocatalytic devices composed basically by water, a photosensitive semiconductor responsible for converting photons in usable chemical energy (H2, O2) and sunlight [2,[4], [5], [6]]. Among several semiconductors used as photoanode, hematite is specially promising due to its good chemical stability in aqueous systems, a very favorable optical gap and abundance on the Earth surface. Despite recent innovations, hematite poor optoelectronic properties involving charge separation/transport processes in solid-solid/solid-liquid interfaces as well as the efficiency on photon/energy conversion are still a barrier for limiting the performance of the material [6,7]. The overall efficiency that controls the process remains in coupled parameters defined as ηcat, which considers the chemical reactions on the surfaces and ηsep, which concerns the bulk separation of the generated charges induced by sunlight [8]. Enhanced electronic properties arisen from modification containing for instance Sn4+ [9,10] and Ti4+ [11,12] foreign elements have shown to be a promising way to address hematite electronic limitation. Lately, dual modifications on hematite photoanodes for instance with metal/nonmetal based materials [13] Cobalt based [14] and nickel based compounds deposited over semiconductor surfaces have been reported to act on passivation of intermediate states and enabling hole collection at lower potentials [[15], [16], [17]].

Although the combination of these elements can be promising, the perspective from literature still shows limitation in fully understand how these foreign elements act in hematite photoanodes. This work presents a strategy to effectively decouple and control the efficiency parameters by a meticulous addition of foreign materials into hematite photoanode. For that, the adopted synthesis method allowed the construction of planar and porous nanocrystalline structures with fine thickness control and good reproducibility [18,19]. In contrast to nanorods based structures, planar polycrystalline assemblies allow a higher density of solid/solid interfaces. Such characteristic is desirable once this work concerns on maximization of the modification effect by addition of foreign elements, key parameters for engineering of interfaces.

Section snippets

Design of the photoanode

Ultrathin, bare and modified hematite photoanodes were synthesized via spin coating deposition of iron based polymeric precursor following a recent report [18,19]. In a typical synthesis, iron based inorganic precursor (iron nitrate: Fe(NO3)3.9H2O, Alfa Aesar, 99.5%), citric acid (C6H8O7, J.T.Baker, 99.5%), and ethylene glycol (HOCH2CH2OH, Sigma Aldrich, 99.8%) are dispersed in water. The solution is warmed up to ~70 °C per 30 min to promote chelation of the Iron ions by citric acid followed by

Non-modified hematite photoanodes

Fig. 1 shows the Scanning Electron Microscopy (SEM) micrographs of the fractured non-modified hematite photoanodes via polymeric precursor method. Cross-sectional SEM images reveal a polycrystalline system with nanometric thickness, ranging from ~25 to 130 nm. Examples are shown for photoanodes H-02 and H-05 in Fig. 1a and b, respectively. As can be seen in Fig. 1, the average thickness increment of all synthesized photoanodes is given by the piling of the grains along the microstructure. Such

Conclusion

In summary, the present work concerns in a strategy to engineer hematite electrodes and isolate modification effects at the photoanodes interfaces. For that, a porous and planar morphology using one polymeric precursors and spin coating procedures enabled us to atomically modify the interfaces and surface for catalysis. The selected modifiers, Sn4+ and Ni/Fe, were carefully characterized allowing the elucidation of the role of Sn4+ as responsible for enhanced electronic transport and creation

Declaration of competing interest

The authors declared that they have no conflicts of interest to this work.

Acknowledgment

The authors gratefully acknowledge support from FAPESP (Grants 2017/11986-5 and 2019/01470-7) and Shell and the strategic importance of the support given by ANP (National Agency of Petroleum, Natural Gas, and Biofuels. - Brazil) through the R&D levy regulation. We also thank Otavio Berenguel from Brazilian Nanotechnology National Laboratory (LNNano) for the X-ray diffraction analysis.

References (32)

  • F.R. Negreiros et al.

    Surface Fe vacancy defects on haematite and their role in light-induced water splitting in artificial photosynthesis

    Phys. Chem. Chem. Phys.

    (2017)
  • H. Dotan et al.

    Probing the photoelectrochemical properties of hematite (α-Fe 2 O 3) electrodes using hydrogen peroxide as a hole scavenger

    Energy Environ. Sci.

    (2011)
  • A.G. Hufnagel et al.

    Why tin‐doping enhances the efficiency of hematite photoanodes for water splitting—the full picture

    Adv. Funct. Mater.

    (2018)
  • M.R. Soares et al.

    Unraveling the role of Sn segregation in the electronic transport of polycrystalline hematite: raising the electronic conductivity by lowering the grain‐boundary blocking effect

    Adv. Electron. Mater.

    (2019)
  • D. Cao et al.

    Cathodic shift of onset potential for water oxidation on a Ti 4+ doped Fe 2 O 3 photoanode by suppressing the back reaction

    Energy Environ. Sci.

    (2014)
  • N.T. Hahn et al.

    Photoelectrochemical performance of nanostructured Ti-and Sn-doped α-Fe2O3 photoanodes

    Chem. Mater.

    (2010)
  • Cited by (13)

    • Solution chemistry back-contact FTO/hematite interface engineering for efficient photocatalytic water oxidation

      2022, Chinese Journal of Catalysis
      Citation Excerpt :

      The reduction of water content after obtaining the polymeric precursor solution enabled more effective viscous control allowing the deposition of multiple layers of hematite with superior performance [27]. An additional modification was carried out to obtain similar performance, but synthesizing a monolayer of hematite processed in a single step [28]. By simply modifying the viscosity of the polymeric precursor solution, a single spin-coating deposition allowed a reduction in the number of hematite-hematite interfaces in thin films up to 130 nm in thickness.

    • All-electrochemically synthesized tin and nickel oxide-modified hematite as photo-electrocatalyst anodes for solar-driven water splitting

      2020, Journal of Catalysis
      Citation Excerpt :

      In fact, pure NiOx is a very stable OER catalyst, but its activity is low when compared, for example, to CoOx or FeOx (FeOx anchored on a matrix that is more electronically conductive, and allows iron to operate in a dynamic process, is one of the most active and stable OER electrocatalysts; which is the case of FeOx/NiOx), as shown recently [7]. Indeed, hematite photoanodes surface modified with NiFeOx was previously reported, showing decreased photocatalytic oxygen evolution onset potential [50,51]. In the present study, clearly, the deposition of the NiOx co-catalyst helps on decreasing the charge recombination, but mainly act on improving the charge transfer rate at the interface, since the NiOx is a hole collector, that assist on catalyzing the OER.

    View all citing articles on Scopus
    View full text